Hi Chris,
The 300D Mercedes is probably the best diesel passenger car ever built. But that does not mean they are trouble free. The engines are great, they will run 300k+ miles if they've been maintained and have not been overheated. (If these engines are overheated, they will run ok for 50k or so miles, then will lose their ring seal on the cylinders most affected by the overheating. The best years were 1985-1987. 1987 is the last year for the 300D, there were no diesels imported in 1988, and the 350 series began in 1989. The 350 series are not nearly as dependable as the 300 series.
To get a machine to last 300k miles you have to build it with tight tolerances. If it's got tight tolerances then it HAS to be regularly maintained. The maintenance records are the first thing you want to ask for when looking at a particular car. The original owners papers have a maintenance booklet which has a place for the servicing shop to stamp when the scheduled services are done. You want this log to be as complete as possible. And keep in mind, the engine is only a small part of what can go wrong with a vehicle. The transmission, front suspension, a/c and climate control systems, door lock & vacuum systems, power steering components all will require work at some time in the car's life. If the previous owner has not already repaired or overhauled these systems, you will be doing it.
There are many other things to watch out for, and I'll be happy to answer any questions by email, but this should get you started. I recommend you find a good mechanic and have a pre-purchase inspection done of any car you're interested in buying. It's easy to get one of these cars that needs more work then the car's worth, especially since they are all getting to the age they are.
BTW, the gas Mercedes will also run 300k miles, and in many cases can be a better deal then the diesels. They are actually cheaper to maintain, and they are cheaper to purchase since they don't bring the premium in the market the diesels bring. The 300E from 1987 to 1991 in my opinion is one of the best cars Mercedes built, gas or diesel.
HTH,
Dave Perry aka karmakanic